Recently, a keyless entry system that is capable of locking and unlocking a door of an automobile, house, etc. without directly touching it, for example by transmitting and receiving signal radio waves, has been put to practical use. To realize the keyless entry system, a coil antenna that can transmit and receive signal radio waves is often used. Also, a coil antenna is often adopted even in a so-called radio-controlled clock that tries to accurately perform time adjustment by means of radio waves. Note that a coil component composed of a magnetic core and a wound coil is favorably adopted in a coil antenna. A system including a coil antenna as a constituent element is also called a coil antenna system.
Here, description is made referring to FIG. 12, with respect to an example of a typical coil antenna used for transmission.
FIG. 12A illustrates an exemplary construction of a conventional coil antenna 100.
FIG. 12B illustrates an example of a magnetic field that is generated when an electric current is applied to the coil.
The coil antenna 100 constitutes a series resonant circuit with a magnetic core 102 formed of a ferritic material, a coil 103 of a conductive wire wound around the magnetic core 102, and a condenser 104 series-connected to the coil 103. The resonance frequency f0 of the coil antenna 100 is determined by this series resonant circuit. Here, a case is assumed that an alternating current with the frequency characteristic corresponding to the resonance frequency f0 is applied to the coil antenna 100. At this time, the coil antenna 100 generates a magnetic flux as illustrated in FIG. 12B to form a magnetic field 105. The coil antenna 100 can transmit a signal wave using the magnetic field 105.
In recent years, the demand for a coil antenna that is capable of transmitting and receiving stable radio signals in a broad frequency range is increasing (in the following description, such demand is also referred to as the demand for making the coil antenna to be broadband). To make a coil antenna to be broadband, it is necessary to apply a strong alternating current of a specific frequency to the coil antenna to generate a strong magnetic field and thereby enable transmission of radio wave signals. Therefore, the range of an allowed characteristic for transmitting and receiving radio wave signals is broadly set. Thereby, even if the characteristics of individual coil antennas vary, they will remain in the allowable range, so that simplification of and freedom in the design concerning manufacture of a coil antenna product can be improved. As a result, it can be tried to decrease the cost of the coil antenna product.
Here, description is made referring to FIG. 13, with respect to band-pass characteristic in the vicinity of the resonance frequency f0 of a coil antenna. In FIG. 13, the vertical axis indicates band-pass characteristic: T of the coil antenna and the horizontal axis indicates a frequency: f of the alternating current applied to the coil antenna.
Generally, to realize a broadband coil antenna, it is effective to “loosen” the band-pass characteristic by adjusting the quality factor: Q value of the coil antenna to a specific value. Here, to “loosen” the band-pass characteristic means that the change width of the band-pass characteristic in the resonance frequency is made smaller. If the band-pass characteristic is loosened, even when the resonance frequency of the coil antenna is deviated from a required resonance frequency, decrease in the band-pass characteristic of the coil antenna can be kept small.
A solid line 106a shown in FIG. 13 represents the band-pass characteristic when the Q value is sufficiently large. The frequency at a peak: T1 of the band-pass characteristic expressed by the solid line 106a accords with the resonance frequency: f0. A broken line 106b expresses the band-pass characteristic when an alternating current is applied to the coil antenna at a frequency f0′ slightly deviated from the resonance frequency: f0 that should be obtained. A solid line 107a represents the band-pass characteristic when the Q value has been adjusted to a specific value. The frequency at a peak: T2 of the band-pass characteristic expressed by the solid line 107a accords with the resonance frequency: f0. A broken line 107b represents the band-pass characteristic when an alternating current is applied to the coil antenna at a frequency f0′ slightly deviated from the resonance frequency: f0 that should be obtained.
At this time, the difference: ΔT1 between the Q value: T1 at a peak of the solid line 106a and the Q value: T1′ of the solid line 106a at the frequency: f0′ slightly deviated from the frequency: f0 is ΔT1=T1−T1′.
Further, the difference: ΔT2 between the Q value: T2 at a peak of the solid line 107a and the Q value: T2′ of the solid line 107a at the frequency: f0′ slightly deviated from the frequency: f0 is ΔT2=T2−T2′.
At this time, from FIG. 13, it is indicated as that ΔT1>ΔT2. That is, it can be said that the decrease width of the band-pass characteristic due to the deviation in the resonance frequency is larger when the Q value is higher, than when the Q value is lower.
Here, description is made referring to FIG. 14, with respect to a configuration example that decreases the Q value of the conventional coil antenna 100. Conventionally, to decrease the Q value, the configuration has been widely adopted in which a resistor element 108 is externally connected in series to the condenser 104 provided to the coil antenna 100. Here, the quality factor: Q of the coil antenna can be obtained by the following formula (I):Q=ω·L/R=2πf·L/R  formula (I)
From the formula (1), it is understood that the Q value can be adjusted by changing either or both of the inductance: L of the coil and the resistance: R.
Meanwhile, if the value of the inductance: L is changed by changing the winding number of the coil, etc., the value of the resonance frequency: f0 of the coil antenna also changes, which is inadvisable. Therefore, conventionally, it has been said that it is desirable to adjust the quality factor: Q of the coil antenna by changing the value of the resistance: R.    Patent Document 1 discloses a conventional coil antenna.    Patent Document 1: Publication of Japanese Patent No. 3735104